1. Field of the Invention
The present invention relates to a process for the production of substitute natural gas (SNG) from carbonaceous materials. More particularly, the invention relates to a process for the production of SNG from a carbonaceous material in which the carbonaceous material is converted to a synthesis gas containing the right proportion of carbon monoxide, carbon dioxide and hydrogen for conducting a subsequent methanation while separately adding a gas stream having a molar ratio (H2−CO2)/(CO+CO2) lower than 3.00 to the methanation section of the plant. More particularly, this stream with molar ratio (H2−CO2)/(CO+CO2) lower than 3.00 is preferably a stream containing carbon dioxide withdrawn from the acid gas removal plant.
2. Description of the Related Art
The low availability of fossil liquid and gaseous fuels such oil and natural gas has revived the interest in developing technologies capable of producing natural gas synthetically from widely available resources such as coal, biomass as well as other alternative fuels such as black liquor, heavy oils and animal fats. The produced natural gas goes under the name substitute natural gas or synthetic natural gas (SNG) having methane as its main constituent.
The process of converting a reactant gas containing carbon oxides (CO2, CO) and hydrogen to methane is commonly referred as methanation and represents a well-known technology which for instance has been used intensively in ammonia plants in order to remove carbon oxides, particularly carbon monoxides from the ammonia synthesis gas due to poisonous effect of carbon monoxide on the ammonia synthesis catalyst.
It is also known to produce SNG from a synthesis gas containing carbon oxides and hydrogen by the passage of such synthesis gas through a methanation section including one or more methanation reactors comprising a fixed bed of catalyst and where the synthesis gas is prepared by for instance gasification of the carbonaceous material.
The methanation process is governed by the reactions: CO+3H2=CH4+H2O and CO2+4H2=CH4+2H2O. Accordingly, methanation should be conducted at conditions that ensure a molar ratio H2/CO in the synthesis gas of 3 or 4. During the production of SNG it is often more convenient to operate with the stoichiometric number M defined by the molar ratio M=(H2−CO2)/(CO+CO2). The value of M in the synthesis gas to the methanation section has to be kept as close to 3.00 as possible. A gas with a value of M=3.00 is said to be stoichiometric, a gas with a value of M>3.00 is said to be over-stoichiometric and a gas with a value of M<3.00 is said to be under-stoichiometric.
The provision of a synthesis gas which is stoichiometric (M=3.00) is normally pursued by passing the gas from the gasification through a water gas shift (WGS) stage upstream the methanation section. During WGS carbon monoxide in the synthesis gas is converted under the presence of water to hydrogen and carbon dioxide. Prior to entering the methanation section the carbon dioxide in the synthesis gas produced in the WGS is normally removed by a conventional CO2-wash, such as the Rectisol or Selexol process. Current methods of achieving molar ratios (H2−CO2)/(CO+CO2) as close to 3.00 as possible in the synthesis gas fed to the methanation section involve also some degree of bypassing of the water gas shift reactor. However, due to fluctuations during operation and the inherent dynamic behaviour of the plant which i.a. imply significant time-lags it is difficult to keep the molar ratio (H2−CO2)/(CO+CO2) of the synthesis gas used as feed gas for methanation close to the ideal value of 3.00, which is critical for the proper operation of the SNG plant. This conveys the problem that even small deviations from this value towards values higher or lower than 3.00 in the synthesis gas manifest itself in reduced quality of the final SNG product, since the product will contain inexpedient surplus of CO2 and H2. For instance, while the SNG product obtained from the methanation of a synthesis gas having M=3.00 may contain only 0.7 vol % H2 and 0.4% CO2, the SNG product from a synthesis gas with M=3.05 may contain 3 vol % H2 and the SNG product of a gas with M=2.95 may contain 2 vol % CO2. Hence, it would be desirable to be able to provide a process which properly controls the ratio (H2−CO2)/(CO+CO2) in order to obtain a final SNG product of constant high quality, i.e. a SNG product after the final methanation stage which contains above 90 volt CH4, particularly above 95 vol % CH4 with deviations of no more than 5%, less than 2 vol % H2 and about 1.1 vol % or less of carbon oxides (CO2 and CO) irrespective of the fluctuations experienced in the plant, particularly in the water gas shift stage (WGS).
According to the prior art the values of (H2−CO2)/(CO+CO2) or H2/CO-ratio are conventionally adjusted by the use of membranes, by WGS followed by CO2-removal, or by splitting streams upstream WGS with subsequent CO2-removal.
Hence, WO-A-2006/09.0218 describes the use of membranes for the forming of hydrogen-adjusted synthesis gas streams during the production of a variety of synthetic hydrocarbons. This patent application is devoted to Fischer-Tropsch synthesis, DME and MeOH applications and to the adjustment of the H2/CO and (H2−CO2)/(CO+CO2) ratio of a synthesis gas produced by steam methane reforming and gasification.
U.S. Pat. No. 4,064,156 describes the methanation of synthesis gas in which the H2/CO ratio is adjusted by using an over-shifted feed gas having a H2/CO ratio above 3 or 4, i.e. above the stoichiometric ratio needed for methanation. Excess CO2 in the feed gas is used as a diluent to absorb the heat evolved in the methanation reactor. Part of the excess CO2 is removed prior to methanation by conventional acid gas wash.
U.S. Pat. No. 4,124,628 discloses a methanation process comprising gasification, optionally water gas shift, CO2-removal and methanation, the latter being conducted in six stages and with CO2 removal in between the 5th and 6th methanation stage.
U.S. Pat. No. 4,235,044 deals with the issue of fluctuations in feed gas rate in continuous operations for the production of methane. The ratio H2/CO is regulated by splitting the syngas stream upstream the water gas shift (WGS) section. Part of the stream not passed through WGS serves to adjust the H2/CO ratio of the WGS treated stream, thereby resulting in a high H2/CO ratio in the gas to the methanation reactors. A purified stream from the gasification may be diverted and added directly to a second methanation reactor with CO2 removal being conducted after this reactor.
WO-A-2008/013790 discloses the conversion of carbon to SNG via steam reforming and methanation. In the acid gas scrubbing (AGS) zone it may be desirable to leave a certain amount of CO2 in the scrubbed stream used as feed gas for methanation depending on the end use of the methane, e.g. as pipeline gas or as raw material for MeOH synthesis.
WO-A-02/102943 discloses a methanation process in which H2 or CO2 are separated from the methane product by use of membranes or pressure swing adsorption (PSA) and in which H2 is recycled to the synthesis gas feed.
Our U.S. Pat. No. 4,298,694 describes methanation of syngas from gasification and purification stages and which is divided in two part streams, one of which is methanised in an adiabatic methanation reactor and subsequently unified with the other part stream. The combined stream is then added to a cooled methanation reactor.